Method and apparatus for preparing an additive for introduction to a gas scrubber

ABSTRACT

A METHOD AND APPARATUS FOR PREPARING AN ADDITIVE FOR USE IN REACTING WITH SULFUR BEARING FLUE GASES IN A WET SCRUBBER. THE ADDITIVE IN THE SOLID STATE IS ENTRAINED IN A GAS STREAM, IS TRANSPORTED TO A SLURRY MAKE-UP AREA, IS CONTACTED WITH WATER TO WET AND DISENTRAIN THE ADDITIVE TO FORM A SLURRY AND SUBSEQUENTLY IS CONVEYED IN SLURRY FORM TO THE WET SCRUBBER WHERE IT REACTS WITH SULFUR OXIDES IN THE FLUE GAS ADMITTED TO THE SCRUBBER. THE ADDITIVE TRANSPORTING GAS, AFTER MOST OF THE ADDITIVE HAS BEEN REMOVED IN THE SLURRY MAKE-UP AREA, IS CONDUCTED TO THE FLUE GAS ADMITTING PORTION OF THE SCRUBBER. IN THE PREFERRED EMBODIMENT OF THE INVENTION, THE ADDITIVE TRANSPORTING GAS IS PROVIDED BY TAPPING OFF A PORTION OF THE UNSCRUBBED FLUE GAS PRIOR TO ITS ADMISSION INTO THE WET SCRUBBER. A COARSE, UNCALCINED ADDITIVE IS PULVERIZED IN A PULVERIZING MILL AND THE FLUE GAS STREAM IS PASSED THROUGH THE MILL TO DRY AND ENTRAIN THE ADDITIVE.

E. K. RICKARD 3,687,613 METHOD AND APPARATUS FOR PREPARING AN ADDITIVEAug. 29, 1972 FOR INTRODUCTION TO A GAS SCRUBBER Filed Oct. 27, 1970 a mWI N AW ww NM 0 N v N W W Nb tmk 3s a 05 .332 .Yll l w wwmw um mw w Wmkzqh @ZTIBMWA lll ll INVENTOR.

o u R A K m R A K. L Q R A W ATTORNEY United States Patent O METHOD ANDAPPARATUS FOR PREPARING AN ADDITIVE FOR INTRODUCTION TO A GAS SCRUBBEREarl K. Rickard, East Granby, Conn., assignor to Combustion Engineering,Inc., Windsor, Conn. Filed Oct. 27, 1970, Ser. No. 84,309 Int. Cl. B01d47/06 US. Cl. 423-242 9 Claims ABSTRACT OF THE DISCLOSURE A method andapparatus for preparing an additive for use in reacting with sulfurbearing flue gases in a wet scrubber. The additive in the solid state isentrained in a gas stream, is transported to a slurry make-up area, iscontacted with water to wet and disentrain the additive to form a slurryand subsequently is conveyed in slurry form to the Wet scrubber where itreacts with sulfur oxides in the flue gas admitted to the scrubber. Theadditive transporting gas, after most of the additive has been removedin the slurry make-up area, is conducted to the flue gas admittingportion of the scrubber.

In the preferred embodiment of the invention, the additive transportinggas is provided by tapping off a portion of the unscrubbed flue gasprior to its admission into the wet scrubber. A coarse, uncalcinedadditive is pulverized in a pulverizing mill and the flue gas stream ispassed through the mill to dry and entrain the additive.

BACKGROUND OF THE INVENTION Recent concern about air pollution hasengendered a growing number of systems designed to remove particulatematerials and gases such as S and S0 from the flue gases discharged fromfossil fuel fired steam generating systems and the like. Many of thesesystems rely on additives introduced into the flue gas stream to reactwith the gaseous sulfur compounds therein in a manner forming sulfurcompounds which may be more easily removed from the flue gas. Thesesystems generally ,use oxides, hydroxides and carbonates of alkali andalkaline earth metals as the additive, with a strong preference forlimestone or dolomite for economic reasons. Most early uses of theseadditives provided for their introduction into a furnace area where theywere calcined and subsequently removed much of the S0 present in thegases resulting from combustion. The combustion or flue gases containingthe partially reacted additive were then further cleaned by passing themthrough a wet scrubber wherein more of the additive is permitted toreact with the gaseous sulfur compounds to form compounds which may besettled out.

More recently, flue gas cleaning systems have been developed whichintroduce the additive to the wet scrubber without first passing itthrough the furnace or combustion chamber. These latter mentionedsystems are not normally used in combination with additive introductionthrough the furnace, but may be done. It has been found that additiveintroduction to the wet scrubber downstream of the furnace is quiteeffective in reacting with and removing the gaseous sulfur compoundsfrom the flue gas without requiring the sometimes diflicult andundesirable introduction of the additive through the furnace. In those'ice arrangements wherein a dust collector is located upstream of thewet scrubber, much of the unreacted additive introduced through thefurnace will be removed from the system before it is available forreaction in the scrubber. Introduction of the additive to the scrubberwithout the pass through the furnace ensures that more of it isavailable for reaction in the scrubber.

While the additive might be admitted to the scrubber as dry particlesentrained in gas, this has certain problems such as the difliculty ofevenly distributing the additive to the scrubber bed and the problem ofbuild-up at the scrubber inlet when the dry additive contacts watervapor from the scrubber.

However, introduction of the additive to the scrubber in a slurry hasproven quite satisfactory. The slurry is easily transported and may besprayed for even distribution to the scrubber bed. Therefore, a needexists for an effective and economic means of preparing the raw additivefor introduction to the wet scrubber in a readily useable slurry.

SUMMARY OF THE INVENTION The invention comprises a method and apparatusfor converting a solid state additive such as limestone or dolomite intoa slurry which may be sprayed or otherwise introduced into a flue gasscrubber associated with a furnace. The additive in the solid state isdelivered to an entrainment chamber and entrained in a confined streamof flue gas passed therethrough. The flue gas stream containing theentrained additive is conducted to a slurry make-up area where it isdirected into contact with water to wet and disentrain the additive. Thedisentrained additive is collected as a slurry and subsequentlyintroduced as needed to the wet scrubber. The gas stream, followingdisentrainment of most or all of the additive, is conducted from theslurry make-up area to the flue gas admitted portion of the scrubberWhere it joins the flue gases to be scrubbed. Precalcining of theadditive permits it to more readily dissolve in water and accordingly toreact at a more rapid rate with the gaseous sulfur compounds than if itwere not calcined. It has been found, however, that by finelypulverizing an uncalcined additive it will react with the sulfur oxidessufliciently fast to make it commercially attractive.

In the preferred embodiment of the invention a gas swept pulverizingmill receives coarse, uncalcined and sometimes moist limestone ordolomite and pulverizes it to a fine powder. A warm gas stream, obtainedby tapping off a portion of the unscrubbed flue gas, is passed throughthe pulverizing mill to accomplish entrainment therein of the pulverizedadditive. The additive bearing gas stream then enters a slurry tankwhere the gas stream is forced, by baflles or the like, to contact waterwhich may exist in the form of a spray or wetted surfaces or both. Theadditive particles, when wetted by the water, leave the gas stream andcollect as a slurry in the bottom of the tank. The Water used to contactand Wet the additive particles serves as the slurry make-up water, andas such is also the make-up water for the scrubber system. The gasstream exits from the slurry tank after contact with the water and isconducted to the flue gas admitting portion of the scrubber where itrejoins the unscrubbed flue gas and is subsequently cleaned in thescrubber. Any

3 additive particles remaining in the gas after it exits from the slurrytank are available for reaction with sulfur compounds in the scrubber.

The invention is particularly appealing for use with oil fired steamgenerating plants which, because of conversion from coal firing, have anidle pulverizing mill available.

BRIEF DESCRIPTION OF THE DRAWING The figure schematically illustrates awet scrubber for cleaning furnace flue gases and the additivepreparation equipment therefore in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, afossil fuel fired furnace as used for the production of heat in steamgencrating units produces large quantities of gases containing theby-products of combustion. These gases are exhausted from the furnace asa flue gas, are transported away from the furnace by conduit 12, and areultimately released to the atmosphere, generally by means of a stack 14.Release to the atmosphere of many of the constituents of the flue gas,such as fly ash and the oxides of sulfur, S0 and S0 is undesirable. Oneof the more effective Ways of removing these undesirable constituentsfrom the flue gas is by scrubbing the gases with water in a wet scrubberand particularly, scrubbing with an additive in the water. The gaseoussulfur compounds are relatively insoluble in water, but their reactionwith an additive in the water, such as dolomite or limestone, is farmore rapid and most of the reaction products are relatively insolubleand may be easily collected and removed, as by settling them out. Thusthe flue gas in conduit 12 is conveyed, usually through an air preheaterT6, to a wet scrubber 18 wherein the above-described scrubbing occurs.Wet scrubber i8 is generally comprised of one or several marble beds 20onto which is sprayed a watery slurry containing an additive such aslimestone or dolomite. Flue gas conduit 12 enters scrubber 13 at a gasadmitting zone 22 of the scrubber upstream of marble beds 20. The gas isscrubbed principally by contact with the additive in marble beds 20 andthrough contact with the additive-bearing sprays. Most of the introducedslurry and reaction products fall to the bottom of scrubber 13 toreactor tank portion 24 thereof where some further reaction between theadditive and the sulfur compounds occurs.

Initial introduction of the additive-bearing slurry to scrubber 18 isaccomplished by sprayers 26 which may be located above or below marblebed 20. Further, the slurry collected in reactor tank portion 24 ofscrubber 18 is usually recycled for most effective use of the additiveand scrubber. Introduction of the recycled slurry is accomplished bysprayers 28 which may be located above or below marble bed 20. Apreferred arrangement places sprayers 26 below bed 20' directed upwardlythereagainst and sprayers 28 above bed 20 directed downwardly thereon.

Ultimate removal of solid wastes from scrubber 18 occurs throughoverflow from the bed 20 into pots 39. The overflow is conducted tosettling tanks where the solid wastes settle out. The scrubbed fluegases are drawn out of scrubber 18 by inducted draft fan 32 whereuponthey pass to the atmosphere through stack 14. Because these gases arenow saturated and may form a visable plume when discharged to theatmosphere, means such as heat exchanger 34 are provided for heating thegases to the extent necessary to avoid plume formation.

Applying the gas scrubbing technique previously described requiresintroduction of the additive to scrubber 18 in the form of a slurry. Themethod and apparatus of the invention provide effective and economicmeans for preparing an additive 36, such as limestone or dolomiteexisting in the solid state, for introduction to scrubber 18 by sprayers26 in the form of a slurry which will readily react with the sulfurcompounds of the flue gas.

Bunker 38 receives additive 36 from a large on-site storage area, notshown, and holds the additive ready for delivery to the slurry make-upsystem. While the scope of the invention is intended to include additive36 existing in bunker 38 in a precalcined and pulverized form, itseconomic advantages are greatest when additive 36 exists in bunker 38 ina coarse uncalcined condition. Additive 36 is delivered, as by agravimetric or volumetric feeder 40, from bunker 38 to a gas sweptpulverizer, such as bowl mill 42. The additive feed rate is determinedby the pH of the overflow efiluent into pots 30. As the pH decreases,the rate of additive delivery to bowl mill 42 is increased. Additive 36as received from bunker 38 would most commonly be in chunks having asize of several centimeters and may have high moisture content if it haspreviously been exposed to the elements. Bowl mill 42 crushes andpulverizes additive 36 to a powder in a wellknown manner.

Bowl mill 42 then serves as an entrainment chamber for the pulverizedadditive 36. A stream of warm gas is passed through bowl mill 42 servingboth to dry and to entrain the pulverized additive 36 in the gas stream.It is necessary that the gas admitted to bowl mill 42 have sufficientvelocity in passing therethrough to efifect entrainment of the additive.This maybe accomplished by a fan located upstream of the mill, or moreconveniently, by exhauster 43 associated with and located downstream ofthe mill. That portion of the additive in bowl mill 42 which is tooheavy because of size or moisture content or both to be entrained by thegas stream continues to be dried and pulverized until entrainmentoccurs.

The additive preparation system of the invention taps oil? a portion ofthe hot flue gas in conduit 12 prior to its entry into scrubber 18 anduses this for the warm gas stream which is passed through bowl mill 42.The figure depicts two points along flue gas conduit 12 prior to itsentry to scrubber 18 at which a portion of the gas may be tapped off.The principal tap occurs at point 44 which is located between airpreheater 16 and scrubber 18. Fine gas temperature at this point will beapproximately 300 F. The flue gas at 300 R, which leaves flue gasconduit 12 at tap point 44 is conducted through gas conduit 45 to thebowl mill 42 where it effects substantial drying and entrainment of theadditive 36 in the bowl mill. Valve 47, located in gas conduit 45, maybe used to regulate the line gas flow from tap point 44 to mill 42 inorder to vary the drying capabilities of the gas in the mill. However,if additive 36 has absorbed considerable moisture, it may also benecessary to increase the temperature of the gas entering bowl mill 42.This is accomplished through the use of a second flue gas tap at a point4 6 along flue gas conduit 12. Tap point 46 is located upstream of airpreheater 16 where temperatures of about 700 F. exist. The hot gastapped at point 46 is conducted through gas conduit 48 and combined withthe gas tapped at point 44 at a junction 50 in gas conduit 45. A valve49 located in conduit 48 is varied, in combination with valve 47, toprovide flue gas temperatures at junction 56- Which are intermediatethose existing at tap points 44 and 46.

Additive 36, pulverized and entrained in the gas passing through bowlmill 42, is exhausted from the mill through exhauster 43 and isconducted through conduit 58 to entry port 60 in enclosed slurry tank62. The gas entrained additive is discharged from conduit 58 to theinterior of slurry tank 62 where water spraying means, such as nozzles64 connected to a source of make-up water, develop a water spray 66positioned to contact the gas-entrained particles. When the particles ofadditive 36 contact water spray 66, they become wetted and accordinglyheavier thereby effecting a disentrainment of the particles from theflue gas transporting them. The disentrained, wetted particles ofadditive combine in the bottom of slurry tank 62 with the water fromspray 66 to form a slurry 68. A mixer keeps slurry 68 well mixed. Thevolume of slurry 68 in tank 62 is controlled by varying the quantity ofmake-up water supplied to sprays 66.

A gas exhaust port 72 is located in the upper portion of tank 62 throughwhich the transporting gas may exhaust. In order to ensure that the gasstream containing additive 36 will come into contact with spray 66,baffles 74 are arranged to conduct the addtive-bearing gas streamthrough spray 66 prior to exhausting at port 72. The bafiles 74additionally force a change in direction of the gas in its passagethrough tanke 62 which serves to further disentrain particles ofadditive 36 from the gas stream. Though only a single water spray anbaflle arrangement has been shown, it should be appreciated that aplurality of such arrangements may exist in tank 62 to increase thecompleteness with which the addative is removed from the gas to formslurry 68. A certain amount of additive disentrainment is also effectedwhen the additive particles contact the wetted surface of bafiies 74 andthe slurry 68.

Slurry 68 leaves slurry tank 62 at slurry exhaust port 76 and isconveyed through conduit 78 to sprayers 26 by means of pump 80. A valve79 in slurry conduit 78 is varied in response to the volume of slurry inthe reactor tank portion 24 of scrubber 18 to control the rate of slurryaddition to scrubber 18 from slurry tank 62.

The gas exhausting from slurry tank 62 at exhaust port 72 will containlittle of the additive 36 with which it entered the tank. However,because the gas exiting at 72 remains an unscrubbed flue gas containingoxides of sulfur and some small portion of fly ash and entrainedadditive, it is conducted through gas conduit 82 to scrubber 18 where itrejoins the unscrubbed flue gases from conduit 12 entering flue gasadmitting zone 22 and is cleaned in the manner previously described. Thesmall amout of additive remaining entrained in the gas is available forreaction with the sulfur oxides in the scrubber.

This arrangement thus permits a relatively inexpensive additive such ascoarse, uncalcined limestone or dolomite which may be exposed to theelements during on-site storage to be used in a wet scrubber. The finelypulverized additive delivered to the slurry make-up tank by the flue gasforms a slurry which will react effectively with the gaseous sulfuroxides in the scrubber and some of the preexisting flue gas iseffectively used as an additive drying and transporting medium and issubsequently routed to the scrubber for cleaning.

While I have illustrated and described a preferred em-. bodiment of myinvention, it is to be understood that such is merely illustrative andnot restrictive and that variations and modifications may be madetherein without departing from the spirit and scope of the invention. I,therefore, do not wish to be limited to the precise details set forthbut desire to avail myself of such changes as fall within the purview ofmy invention.

What is claimed is:

1. In a system for removing sulfur oxides from a flue gas wherein saidflue gas is admitted to a wet scrubber for reaction with an additivetherein, the method of preparing said additive and scrubbing said fluegas comprising the steps of:

(a) feeding said additive in the solid state to an entrainment chamber;

(b) passing a portion of said flue gas through said entrainment chamberwhereby said additive is entrained in said gas;

(c) conducting said gas entrained additive to a slurry tank;

(d) contacting the gas entrained additive with water in said tank to wetand disentrain said additive;

(e) collecting the wetted additive and said water as a slurry in saidtank;

(f) transporting the additive-bearing slurry to said wet scubber forreaction therein with the sulfur oxides in said flue gas; and

(g) conducting said gas from said slurry tank to said scrubber forcleaning therein.

2. The method of claim 1 including the step of pulverizing said additivein said entrainment chamber.

3. The method of claim 2 including the step of varying the temperatureof said flue gas passed through said entrainment chamber to control themoisture content of said additive being entrained.

4. The method of claim 2 including the step of varying the flow rate ofsaid flue gas passed through said entrainment chamber to control themoisture content of said additive being entrained.

5. In a plant having a furnace with a hot flue gas outlet, conduit meanscommunicating said flue gas outlet with a scrubber, said furnaceproducing a hot flue gas containing sulfur oxides and wherein saidsulfur oxides are removed from said flue gas by conducting said gas tosaid wet scrubber, reacting said gas with an additive in said scrubber,and discharging the cleaned gas to the atmosphere, apparatus forpreparing said additive and scrubbing said flue gas comprising incombination:

(a) a source of additive in the solid state;

(b) a gas swept pulverizing mill having an additive inlet, a gas inletand a gas exhaust;

(c) means for delivering said additive to said additive inlet of saidmill for pulverization therein;

(d) an enclosed slurry tank having a gas inlet, a gas outlet and aslurry outlet;

(e) first gas conducting means communicating said gas inlet of saidpulverizing mill with said conduit means upstream of said scrubber forintroducing a portion of said flue gas to said mill;

(f) second gas conducting means communicating with the gas exhaust ofsaid mill and the gas inlet of said slurry tank for conveying gas withentrained additive from said mill to said slurry tank;

(g) third gas conducting means communicating with the gas outlet of saidslurry tank and said scrubber for introducing gas exhausted from saidslurry tank to said scrubber;

(h) means for establishing gas flow of a portion of said flue gasthrough said first, second, and third gas conducting means wherebypulverized additive in said mill is entrained in said gas and carried tosaid slurry tank;

1(i) means in said slurry tank for contacting the gas entrained additivewith water whereby said additive is disentrained from said gas to form aslurry with said water; and

(j) slurry transporting means communicating with the slurry outlet ofsaid slurry tank and said wet scrubber for introducing said additivebearing slurry to said scrubber for reaction with flue gases therein.

6. The apparatus of claim 5 wherein said first gas conducting meansincludes means for varying the gas flow rate to said mill whereby therate of heat input to said mill is controlled.

7. The apparatus of claim 5 wherein said means in said slurry tank forcontacting said gas entrained additive with water comprise:

(a) means for creating a water spray; and

(b) bafile means for directing said gas entrained additive into contactwith said water spray.

8. The apparatus of claim 5 wherein said conduit means includes a heatexchanger and said first gas conducting means comprise:

(a) a first flue gas tap upstream of said heat exchanger for divertingflue gas of a first high temperature;

(b) a second flue gas tap downstream of said heat exichanger fordiverting flue gas of a second temperature lower than said first;

(c) means for combining in varying proportions said flue gas diverted bysaid first and second taps respectively to obtain a gas temperatureintermediate those existing at said first and second taps; and

(d) means for conducting said gas combined from said first and secondtaps to said pulverizing mill.

7 9. The apparatus of claim 8 wherein said means in 2,142,406 1/ 1939Nonhebel et a1. 55-228 X said slurry tank for contacting said gasentrained additive 3,160,352 12/1964 Mollring 23-267 R with watercomprise: 3,320,906 5/ 1967 Domahidy. 1

(a) means for creating a water spray; and 3,617,212 12/ 1971 Shah 23-2SQ (b) baflie means for directing said gas entrained addi- 5 ti-ve intocontact with said water spray. FRANK LUTTER, Primary Examiner ReferencesCited V. GIFFORD, Assistant Examiner UNITED STATES PATENTS U.S. C1. X.R.

1,532,647 4/1925 Bergman. 10 23-267; 55-73, 222, 228, 233; 110-1 P, 119;241-18, 2,066,418 1/ 1937 OMara. 261-3

